As an improvement over the spray painting of articles such as automobile bodies, boats and household appliances, a new kind of paint-coated sheet material (also called paint-coated film) has been developed to provide protective and decorative finishes. The new material comprises a flexible, stretchable, thermoplastic carrier film which has a protective and decorative paint layer of uniform thickness and appearance on one side, and, optionally, an adhesive layer on the other side. It can also have other layers such as a tie or bonding layer between the paint layer and the carrier film. In a preferred embodiment, a transparent topcoat covers the paint layer. This combination provides an attractive basecoat-clearcoat appearance which is especially desirable for automotive finishes.
Using known vacuum thermoforming procedures, the paint-coated film can be stretched and bonded to an article such as an automobile body panel to give a basecoat-clearcoat finish. Advantages of obtaining such finishes by the use of paint-coated film instead of by spray painting include improved control of evaporating solvents, which reduces air pollution, and the elimination or reduction of paint sludge as a hazardous waste product. Furthermore, the new paint-coated film has a remarkably smoother and more attractive appearance than spray painted finishes. This new type of thermoformable sheet material and a process for its manufacture are described in the G. G. Reafler U.S. patent application Ser. No. 116,426, filed Nov. 3, 1987 now abandoned. The process involves applying the paint composition to the surface of the thermoplastic carrier film by laminar flow coating, followed by drying and then coating and drying each additional layer in sequence to obtain a paint-coated film of excellent gloss and smoothness.
The paint-coated film can be stretched and bonded by vacuum thermoforming to a three dimensional substrate such as an automobile panel to form a bonded article. Alternatively, the film can be bonded by simply pressing the film against the substrate surface. Unlike the vacuum thermoforming procedure which is most suitable for attaching a protective and decorative sheet material to a three-dimensional substrate, this procedure is commonly used for covering essentially planar surfaces for "toning and styling" purposes. When the sheet material is used as a toning and styling film, it need not be thermoformable, although flexibility and moderate stretchability are desired. The film is simply stretched along the length of the panel to be covered and pressed by hand into adhesive contact with the panel. The film can be mildly heated to aid in conformance to moderate contours.
Whichever mode of bonding is used, and notwithstanding the film's excellent decorative and protective characteristics, this high quality sheet material can develop objectionable surface defects if the bonded article is subjected to high temperatures over a period of time. A defect that can occur, for example, if an automobile covered with the paint-coated sheet material is exposed to the hot sun on a warm day for several hours, is known as "texturing". This is a roughening of the surface of the paint-coated film or sheet material which detracts from the beauty of the normally smooth, glossy surface. Such texturing can be caused or contributed to by more than one factor but it has now been found that a significant or major factor is the character of the substrate surface to which the adhesive-coated sheet material is bonded. As a result of this discovery, the present invention provides a process for bonding a protective and decorative sheet material to a substrate in a manner which reduces or controls the extent of a subsequent texturing of the surface of the sheet material.
A vacuum forming process for adhesively bonding a thermoplastic sheet to an interior automobile panel is described in U.S. Pat. No. 3,551,232. The objective of this patent is to overcome the problems of bubbling and blistering of the thermoplastic sheet that tends to occur in the vacuum-forming process. It achieves this by use of an adhesive which contains an inert particulate filler for the purpose of minimizing the entrapment of air. The use of an adhesive composition that contains granular particles is not believed to be conducive to obtaining a smooth surface. Furthermore, in the case of exterior automobile panels, it is necessary not only to prevent air entrapment during the bonding operation, but also to avoid later development of surface texturing.
U.S. Pat. No. 3,193,424 to John A. Scott relates to a method of increasing the strength of the adhesive bond formed between metal parts by disposing between the confronting surfaces of the elements to be joined a layer of liquid adhesive containing a finely divided solid abrasive substance, developing a relative movement of said elements to abrade the confronting surfaces, and curing the adhesive to bond the surfaces together.
U.S. Pat. No. 4,778,548 to Joseph R. Fox refers to a method of bonding carbon composite frictional materials to solid substrates with a high temperature adhesive which includes roughening the surface of the substrate by sandblasting, chemical etching, etc. prior to applying the adhesive and subsequent heating.
While the above-cited patents relate to the bonding of adhesive coated films, webs and sheets to a substrate for a variety of purposes, none of them offer any suggestion that a specific combination of substrate roughness and surface energy will prevent texturing of the bonded film, web or sheet under high temperature conditions.
According to Bragole et al., U.S. Pat. No. 4,321,307, the adhesion of polymeric bodies to painted substrates can be enhanced by raising the paint coating's critical surface tension through irradiation. While the patent shows that the adhesion of polymeric bodies to a paint coat can be improved by raising the paint coat's surface tension of wetting through irradiation, such treatment does not prevent texturing of a bonded paint-coated film when subjected to high temperatures, as will be shown hereinafter in more detail.